For a storage device including a plurality of volumes, a technique for improving redundancy of data by duplicating (copying) data stored in a volume (referred to as an operation volume or a copy source volume) to another volume (referred to as a copy destination volume) is widely used. In addition, there is a remote copy technique for duplicating data stored in a volume of a storage device among a plurality of remotely arranged storage devices to a volume of another storage device among the plurality of remotely arranged storage devices.
As such a copy technique, there is a method called equivalent copy (also called full copy) for generating a complete duplicate of an operation volume in a copy destination volume. As the equivalent copy, there are Equivalent Copy (EC) in which a copy source and a copy destination are arranged in the same housing and Remote Equivalent Copy (REC) in which a copy source and a copy destination are arranged in separate housings.
An advanced copy function is supported in some storage devices.
The advanced copy function is a function of causing a storage device to generate a copy by itself at a high speed without using a central processing unit (CPU) of a host device. The advanced copy function copies data stored in a copy source volume at a certain time point to another volume in a short time.
The advanced copy function includes methods such as EC for generating a duplicate synchronized with update of a copy source volume and One Point Copy (OPC) for generating a duplicate of an overall copy source volume at an arbitrary time point. As OPC, there are QuickOPC, SnapOPC, and the like.
QuickOPC is a method in which whole data stored in a copy source volume is first copied to a copy destination volume and thereafter only updated data (differential data) is copied to the copy destination volume. SnapOPC is a method using Copy-on-Write in which only data to be updated is duplicated to a copy destination volume when data stored in a copy source volume is updated. QuickOPC and SnapOPC may reduce physical capacities of duplicated volumes, compared with full backup.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication No. 2002-108571, Japanese Laid-open Patent Publication No. 2003-242011, and Japanese Laid-open Patent Publication No. 2005-242747.
As described above, a volume (SnapOPC duplicated volume (SDV)) duplicated by SnapOPC does not hold whole data stored in an operation volume that is a copy source. Thus, if a hardware failure such as a disk double failure occurs in the operation volume, data stored in the SDV is not useful for recovery of the operation volume.
Thus, in order to prepare against a hardware failure of the operation volume, an equivalent copy (equivalent volume) that is a backup of the overall operation volume is to be generated by EC or REC in addition to SnapOPC.
However, data held in the equivalent volume generated by fully copying the operation volume is data at a time point when EC is started. Thus, if SnapOPC is started after EC is performed, and if a hardware failure of the operation volume occurs, data stored in the operation volume at a time point when SnapOPC is started is not restored.
In order to restore the data stored in the operation volume at the time point of the start of SnapOPC, a method is considered in which an equivalent copy of the operation volume is generated every time SnapOPC is performed. However, if an equivalent volume is generated every time SnapOPC is performed, a large amount of disk capacity is consumed and the cost of an overall storage device increases.